


Don't Forget

by TheAngelofFate



Category: Undertale (Video Game)
Genre: Angst, Dad Gaster, Dadster, Emotionally!Hurt!Sans, Family, Gaster falls into The Core, Gaster loves his sons, Gen, Good W. D. Gaster, Hurt No Comfort, Hurt!Sans, Parent W. D. Gaster, Sans Needs A Hug, Sans is the only one who remembers, The Core (Undertale)
Language: English
Status: Completed
Published: 2020-03-23
Updated: 2020-03-23
Packaged: 2021-02-28 21:34:37
Rating: General Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,201
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23274067
Author URL: https://archiveofourown.org/users/TheAngelofFate/pseuds/TheAngelofFate
Summary: This was it. After so many missteps and mistakes, they were finally going to achieve everything they had worked so hard for. They were going to benefit all the Monsters in the Underground, this was the moment where they'd make history.Thing was, no matter how hard one works, your efforts sometimes blow up in your face.
Relationships: Papyrus & Sans (Undertale), W. D. Gaster & Papyrus & Sans, W. D. Gaster & Sans
Comments: 2
Kudos: 45





	Don't Forget

WD Gaster had been a legend.

Their old man had always looked out for them, half the reason he was looking for freedom was not only to his obligation to the King, his dearest of friends, not only was it towards all monster kind, but to them as well.

And sadly, that’s what got him killed, erased from existence completely. Him and all their other colleges while Sans was left alone, the only survivor and the only one who still remembered. Papyrus had flashes, _déjà vu_ , of their father. Again, Papyrus knew they had a father, obviously, his brother was stupid, he was actually smarter than most monster gave him credit for, and even though Paps couldn’t recall a single detail of their dad.

If only his brother knew, if only his brother could truly fathom, what exactly happened to their father, if Sans could just tell him, then he wouldn’t be so optimistic. If only he could tell him what happened that day, because unlike all the monsters in the Underground, he remembered that day, even though he shouldn’t.

He remembered every single detail of that day was burned into his skull; he saw it almost every time he slept.

That horrible, awful day.

"how are the levels doing?" He had asked his friends who were currently at their stations monitoring the Core’s process and stability.

“Core’s current progress at 73%,” One of his colleges Vardon stated, cat ears perked up but his eyes never leaving the screen as he handed Sans a piece of paper, _“_ There is a small blip in the system but that should disappear with time.”

Even back then Sans knew that small problems could very well lead to something far worse, especially when it came to the type of goals they were trying to achieve.  “wouldn’t want to postpone the big event. Although… better keep an eye on it, just to be on the safe side, you know?” 

“Sure thing.” 

Sans looked over the notes one more time before he teleported to the fridge and got himself a bottle of ketchup, he paused for a moment before grabbing a bag of chisps from the vending machine, and teleported to the Core where he saw his father, standing there, observing his creation, checking and double checking the wires, and pressure levels up close, something he’s done quite frequently,  "what's a physicist's favorite food?" 

He watched his father’s shoulders stiffen, obviously not expecting his sudden appearance. Before he sighed and spoke, _“_ _I can only guess…”_

Sans smiled at him, waiting.

His father sighed once more, with a smile on his face. _“Alright, what is a physicist’s favorite food?_ _”_

Sans snickered and tossed him the bag of chisps.  "Fission chips." 

His dad took a moment to process the joke, his face contorting in thought before he low snort, Sans’ beamed at the expression on his father’s face.  "how was that, G?" 

Gaster adjusted the glasses on his face, taking out a chip and nibbling it before answering, _“_ _Better, keep at it and you’ll be driving your brother insane in no time._ _”_

Sans drank a few sips of the ketchup he grabbed, chuckling.  "well I did learn from the best. I’m positively _pun_ tastic thanks to my old man."

His dad rolled his eyes, opened his mouth to speak but was ultimately interrupted by Goner coming into the room, _“_ Doctor Gaster, Vardon is requesting you, he needs you to look something over, before we proceed.”

Sans glanced at Goner and then glanced at Gaster, then he smirked.  “❽︎☟︎♏︎⍓︎ ☝︎✍︎ 🕈︎♋︎■︎■︎♋︎ ♑︎□︎ ⧫︎□︎ ☝︎❒︎♓︎●︎●︎♌︎⍓︎❼︎⬧︎ ●︎♋︎⧫︎♏︎❒︎✍︎” 

His dad and Goner blinked, caught off guard by his change in font. But while Goner looked confused, Gaster let out a laugh,

“✋︎ ⬥︎□︎◆︎●︎♎︎ ♌︎♏︎ ♎︎♏︎●︎♓︎♑︎♒︎⧫︎♏︎♎︎📪︎ ❍︎⍓︎ ⬧︎□︎■︎📬︎” 

Goner let out an exasperated sigh, “Really? Again? You know I can’t understand you when you both talk like that.”

Sans looked directly at Goner and winked,  “🕈︎♒︎♋︎⧫︎🕯︎⬧︎ ⬥︎❒︎□︎■︎♑︎📪︎ 🙵♓︎♎︎✍︎ ♍︎♋︎■︎🕯︎⧫︎ ⬧︎♓︎♑︎■︎ ⬥︎♓︎⧫︎♒︎ ⧫︎♒︎♏︎ ♌︎♏︎⬧︎⧫︎ □︎♐︎ ⧫︎♒︎♏︎❍︎✍︎” 

Kid frowned at Sans, and let out a groan, turning away from them, “ _Ugh_ , that’s it I’m leaving.”

Gaster cleared his throat, trying very hard to hold back another laugh before, speaking again, _“_ _Please, forgive me, Goner, I’m coming, I’m coming.... Sans? Are you joining us?_ _”_

Having finished his bottle of ketchup, Sans placed his hands in his pockets and began walking,

"Yeah, I’m comin’.”

Then an odd smell made him paused, he sniffed the air, and was hit with an odor of gas and burning metal, and it genuinely confused him, because this wasn’t here before, this decent smell just burning, it filled every boney part of him, looking downwards, he saw something in the light below, something that hadn’t been there before. He looked downward, past the scaffolding, and saw something, something odd.

“ _Sans?_ ” His father’s voice broke him out his spell, he looked up out of instinct and saw that he had paused long enough for Gaster and Kid to get a great few distance away from him. “ _What is it?_ ”

Then an explosion erupted, dust from the cave fell from the ceiling, the Core seemed to explode with a burst of light as the ground shook from the intensity catching Sans off balance, making him fall to his knees.

Tardon came rushing out standing in the doorway. “The Core’s destabilizing, we have to find a way to shut it down or all risking losing the entire Underground!”

But it seemed that Gaster wasn’t focusing on that, all Sans saw him doing was running forward, running towards him, “ _Sans!_ _”_

Sans turned and saw Gaster and several other of their colleges running towards him. The fear in Gaster’s voice, the genuine terror on his face was enough to set a chill down his spine, was enough for him to run forward, the name slipping out without his meaning to. 

"Dad!”

Another tremor erupted, as static filled the air, along with a deep piercing white noise. And then pain, an unbelievable jolt of agony exploded from his right eye as lightning and static coming from the corrupted Core. A scream of complete agony tore itself from his nonexistenting throat, as he fell to the scaffolding, clinching his eye. “ _Sans!_ ” Vision blurred, Sans was able to make out the silhouette of his father, lifting him up, a look of terror a crossed his features as he cupped the side of his face. “ _Oh gods, no…._ ”

He curled into Gaster’s arms, fourteen years old and feeling like a child again,

"Dad…?"

His father held him tightly, “ _We need to go._ ”

Sans’ skull was throbbing, making it hard to concentrate, to focus properly.

"But… the Core, everyone else?"

Gaster looked at him, the expression on his face clear that his father was conflicted and lost, for the first time not knowing what to do with a crisis like this. They either run away and risk everyone’s safety or they try to shut the Core down and risk their lives. Sans, even in the state he was in, knew that this couldn’t be an easy decision for his dad to make.

But unfortunately, neither of those choices were one that Gaster had a chance to make, because it seemed that the Core made its own choice. For a split second a chunk of the ceiling broke away from the cave at that exact moment, falling straight down towards them, the size of it casting a shadow in it’s wake. They both simultaneously looked at each other, and then his father placed his forehead up against his and tossed them both away from danger, sending each other in opposite directions as the rock came crashing down onto the scaffolding, the force of the massive weight caused the metal to crack under the pressure, taking not only the bridge with it as it fell towards the Core, but his father along with it.

And despite the pain he was in, despite his blurred vision and his no doubt permanently damaged eye, Sans rushed forward and managed to grasp Gaster’s wrist, stopping the man from being swallowed up by the Core, and desperately trying to pull him back up.

“H-Hang on, G!”

Then a shrill agonizing scream to his left, turning he saw Tardon and Kid being knocked back, backward into the Core, and Sans could only watch in horror as his friends and colleges where there one moment and then gone the next.

“Tardon, Kid!, no!”

“ _Sans!_ ” His father voice seemed to echo throughout the cavern, making Sans pause to look at him, as the wind rushed past them both, Sans did his best to pull his father up, but he was already too weak, and with his right eye as damaged as it was, there was no way he’d be able to teleport them both far enough away from the Core in time. “ _Sans! Let go!_ ”

His soul almost skipped a beat at that suggestion. Sans shook his head, frantically,

“No!”

Because no, he wouldn’t do it, the thought of letting go of his father, watching him fall, watch as he lost him completely.

“No!”

But the reality of it, he knew they didn’t have much time, the gravity produced from the Core was not only now making it hard to move but it seemed that it was actually pulling Gaster towards it. _“Sans, let go! I won’t let you get dragged into my mistake, please!_ ”

Tears formed in his eyes, right eye burning from the liquid and he clung to his father’s wrist, desperately holding on to him.

“No! I won’t, dammit! Please! _Just_ —!”

“💧︎♋︎■︎⬧︎📪︎ ●︎♓︎⬧︎⧫︎♏︎■︎📬︎📬︎📬︎”  His father’s font catching him off guard as he looked to see the man smiling sadly up at him, as he reached upward and gripped his smaller hands.  “✋︎ ●︎□︎❖︎♏︎ ⍓︎□︎◆︎◻︎ ■︎□︎ ❍︎♋︎⧫︎⧫︎♏︎❒︎ ⬥︎♒︎♋︎⧫︎📪︎ ♓︎ ●︎□︎❖︎♏︎ ⍓︎□︎◆︎ ♋︎■︎♎︎ ⍓︎□︎◆︎❒︎ ♌︎❒︎□︎⧫︎♒︎♏︎❒︎ ⬧︎□︎ ❍︎◆︎♍︎♒︎.” 

The declaration, let him stunned him, and that slight pause was enough for his dad to wrench himself out of his grip, Sans felt like his entire soul would cease in the moment, as he watched his father fall into the Core, with a smile on his face.

“NO!”

Then everything around him was engulfed in a bright light, as it blinded all his senses. He could no longer see or hear or even _think_ , all that he could do was succumb to this light. And then he woke up somewhere else, scratch that he woke up someplace else. For the first few minutes, all he could do was lay there, still in pain, still in shock as he opened in eyes and found himself in Waterfall.

Sans blinked several times, staring up at the ceiling, numb from his skull right down to his phalanges. Then seconds later, he felt them all wake up simultaneously, his soul tingled as well as his other bones in his body. And as his body woke up, so did his thoughts. It didn’t take long for the events of what happened to replay, didn’t take him long to think what had gone wrong as the last words from Gaster echoed in his head over and over again.

And if that didn’t make his soul ache against his rib cage, if that didn’t make him feel broken, shattered beyond repair as he laid there in the grass letting the dew soak into his clothes. Somehow Sans remembered stumbling home, where he knew Papyrus was waiting with Grillby. Gods, what was he going to tell him? How was he even possibly going to tell his brother that their father, and the colleges that have come to love him like a family ever since Gaster first made them, were gone?

Taking a breath, he opened the door and only got a few inches inside the door before Papyrus was barreling into him.

"Sᴀɴs! Yᴏᴜ’ʀᴇ ʜᴇʀᴇ! Tʜᴀɴᴋ ɢᴏᴏᴅɴᴇss!”

Then a second later, Grillby came up behind him and with an equally worried expression on his face. “Sans, we’ve been looking everywhere for you. We—” Grillby paused then, as his eyes widened in shock, bringing a hand to the side of his face, “Sans, your eyes— your _bleeding_ …? What in the world happened?”

Sans blinked away the blood from his eye as he tried to speak, tried to voice the reality of what had happen in Hotland just a few moments ago, but all that came out was a low sob, as he hugged Papyrus closer.

"Bʀᴏᴛʜᴇʀ?"

Papyrus was still so young, so innocent and pure. He didn’t deserve to have his whimsy shattered with what he was about to say. Tears spilled down his cheeks bones,  “Oh Pap… Grillz, I’m so sorry, but dad… He’s— he’s _gone_ …” 

Sans had expected for all the worst-case scenarios, on his way here. Crying, screaming, throwing stuff. What he had not been expecting was Papyrus giving him a confused look. “Wʜᴀᴛ ᴀʀᴇ ʏᴏᴜ ᴛᴀʟᴋɪɴɢ ᴀʙᴏᴜᴛ, Sᴀɴs? Gʀɪʟʟʙʏ’s ʀɪɢʜᴛ ʜᴇʀᴇ?”

It was like the rug that was barely holding Sans up from the pit he found himself hovering over was suddenly ripped out from under him. He felt and saw his vision blur, felt nauseous even though he knew that wasn’t possible seeing as he didn’t have a stomach to be nauseous with. He couldn’t process this; he genuinely couldn’t process what his brother just said. Grillby was their father? But that wasn’t right, their father was WD Gaster, the Undergrounds greatest scientist, how could Papyrus even say that, was he teasing him, trying to get him back for all his bad puns with a cruel on of his own?

But…

But the look on Papyrus’ face. The confusion and genuine concern. It made Sans realize that his brother wasn’t teasing him. He truly didn’t know who he was talking, and by the look on Grillby’s face, he had no idea either, neither of them remembered Gaster.

But if they didn’t remember him, then did that mean that no one else did either?

_Why?_

He didn’t understand….

He didn’t want to understand, all he wanted was his father back, his soul ached, his head hurt, and he felt absolutely miserable.

Breaking away from them both, he shook his head. Because, no. He refused to believe it; he couldn’t believe it. Wasting no time, he ran out of the house and frantically began asked around, asking anyone he came a crossed that knew the scientist called Gaster, he looked up notes, and nothing. No one had ever heard of him, not a single solitary monster in the Underground remembered WD Gaster. It was as if his dad had been completely erased from the universe.

Hell, he even went back to the lab to find proof that his father had in fact, existed and nothing. All the records and files they had written down and stored away. All the prototypes he had made along his father and friends were either gone or burned beyond recognition. And then something hit him, because maybe if it had been a distance thing, then maybe, maybe he did have something, some proof that his dad existed. Teleporting back to his room, Sans practically tore his room apart looking for it, but then eventually he found it, a small charred photo consisting of all their colleagues, like Tardon and Kid. And there standing right next to Sans, was Gaster. Sans let out a hysterical laugh that turned into a broken sob. Even though he had just lost them only a few moments ago, to Sans it felt like they had been gone for years, for decades. Then the door to his room burst open and in walked Papyrus and Grillby.

“ _Sans!_ ” They both yelled in unison.

Sans looked at them, tears falling down his eye sockets, he smiled rushing towards them,  “see, bro, look! That’s dad, he’s right here with Tardon and Kid and everyone, remember?”  He said his voice cracking on the last part as he held out the photo to Papyrus and Grillby. And all the hopes that Sans had was dashed when neither of them showed any looks of realization, of remembrance but simply looked back at him with more confusion.

“Sᴀɴs… ʙʀᴏᴛʜᴇʀ, ᴛʜɪs ɪs ᴀ ᴘɪᴄᴛᴜʀᴇ ᴏғ ᴛʜᴇ ᴅᴀʏ Gʀɪʟʟʙʏ ᴀᴅᴏᴘᴛᴇᴅ ᴜs."

Grillby reached out and cupped his cheekbone once again, his expression concerned but confused, “My sister found you both wondering out in Hotland, and brought you both to me, after a few weeks I adopted you, remember?”

“w-what?” 

Sans looked back at the photo expecting to see it change or something, but it stayed exactly the way it was, still displaying Gaster, still showing the bright smiling faces of everyone else in the photo. Sans could see it, see what really was there, but Grillby and more importantly Papyrus couldn’t remember their own father, couldn’t remember all the great thing Gaster did, couldn’t remember the friends they had made back in the Royal Lab, none of it.

So then why could Sans?

Was it because he had survived the ordeal? Was it because he had been transported somewhere else and not fallen into the Core was the reason, he retained all his memories?

Then he touched his left eye, realizing that though the blood faded long ago, the pain was still there. But now along with the pain, was now a deep burning sensation. Was it… Was it because of that? Did the blast to his eyes from the Core made it so he was immune to the effect of the Core?

Whatever the reason, Sans knew that none of it made any difference, his father was still gone, his friends were still gone, and Sans was still here, the sole survivor. All their hard work and effort all this time had been pointless. That numbness filled him once again, that shattering feeling engulfed his soul and stayed there as the adrenaline that had been coursing through him had now vanished. Black spots filled his vision, and he hit the ground before Papyrus or Grillby could catch him.

Later on, after Grillby and his brother finally went to sleep after fussing over him for nearly an hour. Sans cleaned out the back area of their house and brought several things from the lab, scraps that could still work if they were fixed properly. He took out the photo he had and a marker, there he wrote a small note to himself, just two simple words, as his father’s last words echoed through his head.

“i love you too, dad… And i swear i’m gonna get you back.”

**Author's Note:**

> So I don’t know what brought me to write this, seeing as this concept is years late, but I’ve always loved Undertale and more specifically Gaster and more specifically Gaster being a Dadster to Sans and Papyrus. Again I know I’m years late to the party but I just wanted to add my own Dadster story to the mix. 
> 
> Anywoo, I hope you all enjoy this random story, feedback is always welcomed and appreciated down in the comments and I will see you all later.


End file.
